Comparison of the excitatory and inhibitory effects of angiotensin and vasopressin on mammalian portal vein.

Angiotensin produced inhibitory and excitatory responses in guinea pig portal vein and excitatory responses in rabbit and rat portal vein. Vasopressin produced inhibitory responses in guinea pig and rabbit portal vein and excitatory and inhibitory responses in rat portal vein. In guinea pig portal vein, inhibition by angiotensin and vasopressin was associated with an increase in membrane potential which was, on average, smaller than that produced by isoprenaline, and with a suppression of spike discharges. Tachyphylaxis of the responses to angiotensin and vasopressin in portal vein was not observed and their effects were unaffected by tetrodotoxin, atropine and phentolamine plus propranolol.